1. Field of the Invention
The present invention relates to a suspension apparatus of an automotive vehicle.
2. Description of the Related Art
A suspension apparatus of an automotive vehicle, as is generally called a passive suspension, has a damper unit comprising a hydraulic damper and a spring (generally called a coil spring), and suspension characteristics are equally set on the basis of predetermined characteristics of the damper unit. Although a damping force of the hydraulic damper can be variable, this cannot change the suspension characteristics to a large extent.
Recently, a suspension apparatus, as called an active suspension, has been proposed, which allows a change in the suspension characteristics in an arbitrary fashion. This active suspension is basically designed so as to control the suspension characteristics by controlling the supply and the discharge of an operating liquid to or from a cylinder unit interposed between a sprung weight and an unsprung weight.
In the active suspension, the suspension characteristics can be changed to a great extent by supplying or discharging the operating liquid from or to the outside, in order to perform a variety of controls such as ride height control, roll control, pitch control, etc.
Hence, the active suspension requires a circuit for supplying and discharging the operating liquid to and from its cylinder unit, whose supplying and discharging circuit in turn comprises at least a reservoir tank for storing the operating liquid, a pump for pumping up the operating liquid stored in the reservoir tank, a high-pressure line for supplying the operating liquid of a high pressure forced out from the pump to the cylinder unit, and a low-pressure line for returning the operating liquid in the cylinder to the reservoir tank. Further, the high-pressure pressure line is provided with a control valve for supplying the operating liquid while the low-pressure line is provided with a control valve for discharging the operating liquid, thereby permitting a control of the supply and the discharge of the operating liquid to and from the cylinder unit and, as a result, a control of the posture of the vehicle body.
It can be noted herein that, if the operating liquid of a high pressure remains within the high-pressure line when an ignition switch is turned off, there is the possibility that the cylinder unit may be operated to some extent due to the operating liquid of a high pressure left within the cylinder unit. For instance, if some of the operating liquid leaks from the control valve for supplying the operating liquid, the operating liquid of a high pressure is supplied to the cylinder unit corresponding to the control valve from which the operating liquid leaks while the vehicle is parked, and consequently the height of the vehicle body may be partially elevated.
Furthermore, the way of competing with the situation during a trouble in which no active control can be performed in a normal way is one of the problems which are sought to be solved. Therefore, the measure to be taken in that situation has been desired from a fail-safe standpoint.